1. Field
Example embodiments relate to a reflective photomask, a method of fabricating the same, and a reflective blank photomask.
2. Description of the Related Art
The development of highly integrated semiconductor devices may depend on uniformly and finely forming patterns having line widths of less than a sub-micrometer. This fine pattern forming may depend on the patterning ability of the photolithography process. The patterning ability of the photolithography process may be improved by various parameters (e.g., the wavelength of light, photolithography equipment, and the experience of the operator in performing the photolithography process).
A pattern resolution representing the ability of finely forming patterns may be expressed as the following equation: R=k(λ/NA), where R=resolution, k=process factor, λ=wavelength of light, and NA=lens numerical aperture of photolithography equipment.
The lower the resolution (R value) is, the better the fine pattern forming ability may be.
Referring to the above equation, the pattern resolution may be proportional to the process factor and the wavelength of light, and inversely proportional to the lens numerical aperture of the photolithography equipment. The process factor may be the extent or ability of improving the photolithography process by an operator performing the process, a process recipe, and/or etc. The wavelength of light, λ, may be used as an exposure source. For example, i-line light, having a wavelength of about 365 nm, KrF light having a wavelength of about 248 nm, and ArF light having a wavelength of about 193 nm have been used. NA may be the lens numerical aperture of the equipment for performing the photolithography process. As the aperture of the lens is increased to collect as much light having pattern information as possible, the resolution may be improved.
The process factor, k, and the lens numerical aperture, NA, have almost reached the limitation of a current wavelength band of light, and only slight improvements may be had. As such, research has been conducted in an effort to decrease the wavelength of light.
Recently, the photolithography technique using EUV (Extremely Ultra Violet) light has been studied and may be known to be commercially feasible. The EUV light may be referred to as a soft X-ray, and may be a microwave having a wavelength of only about 13.4 nm. Accordingly, it had been expected that EUV light may improve the fine pattern forming ability. However, because EUV light may be absorbed by most media, including air, the established transmission photomask may not be well suited for the photolithography process in fabricating a semiconductor device. However, a photolithography technique using a reflective photomask may be used instead.
Because the reflective photomask may reflect more than 90% of EUV light that may be irradiated, the energy efficiency may be good and the fine pattern forming ability may be excellent. Accordingly, a reflective photomask may be the next generation photomask technique. However, the photolithography process using the reflective photomask may be limited due to structural defects of the reflective photomask.
A reflective photomask will now be more fully described with reference to FIG. 1. FIG. 1 is a cross-sectional view illustrating a reflective photomask according to the related art.
Referring to FIG. 1, the reflective photomask according to the related art may include a reflective layer 20 on a substrate 10 and an absorption pattern 30 on the reflective layer 20. The reflective layer 20 may be formed on the substrate 10 and may reflect incident light. The absorption pattern 30 may keep pattern information to be transferred onto a wafer, and may absorb rather than reflect incident light.
In the reflective photomask, light may not enter vertically and be reflected at the surface of the photomask, but may enter diagonally at an angle (for example, about 1° to 6°) and be reflected. Therefore, the phenomenon (left side) of the incident light not arriving at the reflective layer 20 due to the absorption pattern 30 may occur, and the phenomenon (right side) of the reflected light at the reflective layer 20 not being reflected due to the sidewall of the absorption pattern 30 may occur. Even though an undesired reflection of light may occur at the sidewall of the absorption pattern 30 (not shown), light diffracted at the edge of the absorption pattern 30 may be transmitted onto the wafer.
Because light may be diffracted from the surface and the inside of the reflective layer 20 to transmit the pattern information, the remaining portion of the diffracted light may be essential. However, as illustrated on the right side of FIG. 1, if a portion of the diffracted light is not transmitted onto the wafer, due to the different thickness of the absorption pattern 30, the pattern on the photomask may not coincide with the pattern on the wafer. The pattern information and focus may be moved onto either the photomask or the wafer, and distorted pattern information may be transmitted.
These phenomena cause a shadow effect such that the pattern image information may not properly be transmitted onto the wafer. As a result, the pattern vertical to and the pattern parallel to an irradiating direction of light may have different line widths, and one problem may be that the focus of light irradiated onto the wafer may shake.
The aforementioned defects may be influenced by, for example, the material, slope, height, and pitch between the patterns of the absorption pattern 30 formed on the surface of the reflective photomask. In order to overcome the material problem of the absorption pattern 30, studies have been conducted to develop an absorber of various materials. In order to overcome the slope problem of the absorption pattern 30, much effort has been made to improve the fabrication process of the photomask.
However, the defects caused by the structural characteristics of the reflective photomask may not be remedied by an improvement in the process and in the materials. As long as the absorption pattern 30 has a different thickness, it may be difficult to reduce or prevent the loss of light, which may be diagonally incident at the edge and at the sidewall of the absorption pattern 30.